Courtney Francisco, a reporter for KFOR TV, was one of the witnesses to the execution, but she said she realised there was something wrong shortly before the blinds were closed

A news reporter has described the moments before officials realised something had gone wrong during the execution of killer Clayton Lockett.

Courtney Francisco, a reporter for KFOR TV, was in an adjoining room to witness the event but says that 16 minutes after the execution began, a blind was closed.

Speaking to the BBC she described who Lockett said he had no final words and when the blinds were first opened he was already on the gurney.

At 6.23pm the lethal injection began, Ms Francisco said, and seven minutes later he appeared to be going unconscious.

She said: "The woman prison official said 'go ahead to check and see if he is unconscious' and at that point the doctor said 'Mr Lockett is not unconscious' and that's when Lockett said himself 'I'm not'.

"The old way that they did the lethal injections lasted six to 12 minutes.

"At 6.33pm, three minutes later, they asked if Mr Lockett is unconscious again, the doctor got up and he said he is unconscious. At that point they started administering the two other drugs.

"One stops your breathing, the other one stops your heart. At 6.36pm he stared moving his head around side to side. His head slightly raised off the gurney.

"He mumbled something at that point and at 6.37pm a minute later he started moving his arm, his legs, mumbling. At that point he was actually raising off the bed as if he was trying to talk.

"At 6.38pm he was breathing very hard and the woman prison official looked up, out at the window. She had been staring at him the whole time. At 6.39pm he was trying to talk some more, moving, he said 'Man!', and she said 'we're going to go ahead and close these blinds and halt it temporarily'."

What happened next, Ms Francisco says is that an official spoke to them later on, confirming that Lockett had died from a massive heart attack.

David Autry, one of Lockett's lawyers, said: "This was botched, and it was difficult to watch."

Witness Ziva Branstetter told broadcaster MSNBC Lockett was thrashing about and appeared to be in pain.

"His body was sort of bucking. He was clenching his jaw. Several times he mumbled phrases that were largely unintelligible," she said.

Lockett, 38, was convicted of first-degree murder, rape, kidnapping and robbery for a 1999 crime spree with two co-defendants.

He was found to have shot teenager Stephanie Nieman and buried her alive in a shallow grave where she eventually died.

The execution of fellow inmate Charles Warner, which had been scheduled to take place two hours later, was postponed for a fortnight.